For a slim, high image quality, and low power consumption display device, an organic EL display device is known. An active matrix-type organic EL display device has a plurality of two-dimensionally arranged pixel circuits each including an organic EL element and a drive transistor. The organic EL element is a self-light emitting type electro-optical element that changes in luminance according to a drive current. The drive transistor is provided in series with the organic EL element, and controls the amount of drive current flowing through the organic EL element, according to a gate-source voltage thereof.
In general, a thin film transistor (hereinafter, abbreviated as TFT) is used as the drive transistor in the pixel circuit. Specifically, an amorphous silicon TFT, a low-temperature polysilicon TFT, an oxide TFT (also called oxide semiconductor TFT), or the like, is used as the drive transistor. The oxide TFT is a TFT in which a semiconductor layer is formed of an oxide semiconductor. For example, indium gallium zinc oxide (In—Ga—Zn—O) is used for the oxide TFT.
In general, the gain of the transistor is determined by mobility, channel width, channel length, gate insulating film capacitance, etc., and the amount of current flowing through the transistor changes according to gate-source voltage, gain, threshold voltage, etc. When a TFT is used as the drive transistor, variations occur in threshold voltage, mobility, channel width, channel length, gate insulating film capacitance, etc. If variations occur in the characteristics of the drive transistor, then variations occur in the amount of drive current flowing through the organic EL element. Due to this, variations also occur in the luminance of pixels, degrading display quality.
In view of this, conventionally, there is devised an organic EL display device that compensates for variations in the characteristics of a drive transistor. Patent Documents 1 to 4 and Non-Patent Document 1 describe organic EL display devices that perform only threshold voltage compensation. Patent Documents 5 to 9 describe organic EL display devices that perform both threshold voltage compensation and gain compensation (mobility compensation).
Patent Document 8 describes an organic EL display device including a pixel circuit shown in FIG. 33. The pixel circuit shown in FIG. 33 includes an organic EL element L0, a drive transistor DR, two control transistors SW1 and SW2, and a capacitor Cst. When a scanning signal GL is at a high level, the control transistor SW1 is turned on and a fixed reference voltage Vref is provided to one end of the capacitor Cst. Patent Document 9 describes an organic EL display device that performs both threshold voltage compensation and gain compensation on a per pixel circuit basis, using correction data obtained for each pixel circuit which is stored in a memory.